Systematic Evaluation of Alternative Acid System for High-Temperature Carbonate Formation

Abstract

The dissolution pattern created by injecting acid into carbonate formation for well stimulation can be characterized as one of three types: compact dissolution, wormholing, and uniform dissolution. The dissolution pattern depends on interstitial velocity of acid, which is related to injection rate. Compact dissolution is created at low injection rates, wormholing at intermediate rates, and uniform dissolution at high injection rates. Out of these dissolution patterns, wormholing is considered the most desirable for efficient stimulation of carbonate formation. In a typical wormhole efficiency curve, there exists an optimal interstitial velocity that requires the minimum amount of acid to propagate wormhole to a certain distance. The wormhole structures change from large-diameter at low interstitial velocity (compact dissolution), to well-controlled diameter at optimal interstitial velocity (wormholing), and to more branched pattern at high interstitial velocity (uniform dissolution).

Conventionally, hydrochloric acid (HCl) has been used as the preferred acid system for carbonate acidizing treatment. Alternative acid systems have been developed to overcome some of the weaknesses of HCl and to improve stimulation efficiency. Such acid systems are of particular interest for high-temperature carbonate reservoirs because HCl shows very high reactivity and corrosiveness at high temperatures. Some of the advantages of alternative acid systems are better wormhole efficiency at lower injection rates and less corrosiveness to protect tubulars and surface equipment. The performance of alternative acid needs to be evaluated at high temperatures for the feasibility of being and effective stimulation fluid. The purpose of this study is to experimentally evaluate the performance of an alternative acid system over a wide range of temperatures up to 300°F. The goal is to confirm the applicability of the acid system and identify the optimal conditions for field operations.

Core flood experiments are conducted using Indiana Limestone and Silurian Dolomite cores of 1.5-inch diameter and varying length in this study. Porosity and permeability are measured before acid flooding. The temperature range for experiments is from 125°F to 300°F. Wormhole efficiency curves are generated by running 3-5 experiments at different interstitial velocities for each temperature. Optimal conditions at each temperature are determined from experimental results. The cores are CT scanned after each test to visualize the wormhole structure created. The alternative acid system exhibits very efficient performance with a dominant wormhole created at each test condition. This indicated that the alternative acid system can be used as a stimulation fluid for carbonate reservoirs over a broad range of temperatures up to 300°F. Many studies have observed that wormhole efficiency decreases with increasing temperature for HCl. This is because of the increased reactivity of acid at high temperatures, which results in high optimal interstitial velocity. Similar behavior is observed with alternative acid systems in this study. The performance of alternative acid is also compared with that of 15% HCl at a particular interstitial velocity. The results show better stimulation efficiency for alternative acid systems as compared to HCl.